1. Technical Field
The present invention relates to an in-plane switching (IPS) mode or fringe field switching (FFS) mode transverse field type liquid display panel, and more particularly to an IPS or FFS mode liquid crystal display panel that is equipped with a device for preventing burn-in arising from the voltage that is applied to the scan lines.
2. Related Art
Recent years have seen widespread use of liquid crystal display panels not only in telecommunications equipment but in electrical equipment in general. The liquid crystal display panels that have long been in use are made up of a pair of substrates of glass or the like with electrodes and so on formed on their surfaces, and a liquid crystal layer formed between such pair of substrates. Images of various types are displayed by the application of voltage to the electrodes on the two substrates, which rearranges the liquid crystals, altering the transmittance of light therethrough. This is what may be called the “longitudinal field type”. Among such longitudinal field type liquid crystal display panels there exist those with a twisted nematic (TM) mode or vertical alignment (VA) mode, which however have the problem that their viewing angle is narrow. Accordingly, longitudinal field type liquid crystal display panels with various improvements such as multidomain vertical alignment (MVA) mode have been developed.
On the other hand, among IPS mode liquid crystal display panels there are also known those that may be called “transverse field type” and that differ from the longitudinal field type described above in having electrodes on one substrate only (see JP-A-10-319371 and JP-A-2002-131767). The operating principles of such an IPS mode liquid crystal display panel will now be described using FIGS. 10 to 12. FIG. 10 is a schematic plan view of a single pixel portion of an IPS mode liquid crystal display panel, FIG. 11 is a cross-sectional view along line X-X in FIG. 10, and FIG. 12 is a cross-sectional view along line XI-XI in FIG. 10.
This IPS mode liquid crystal display panel 50A has an array substrate AR and a color filter substrate CF. The array substrate AR has multiple scan lines 52 and common wires 53 provided parallel with one another on a surface of a first transparent substrate 51, and multiple signal lines 54 provided in the direction crossing over these scan lines 52 and common wires 53. In the central portion of each pixel there is provided a common electrode 55 having for example a comb-like shape as in FIG. 10 and extending strip-like from the common wire 53. A pixel electrode 56, likewise of a comb-like shape, is provided so as to enclose the spaces around the peripheries of the common electrode 55, and the surface of the pixel electrode 56 is covered with, for instance, a protective insulator 57 of silicon nitride and an alignment layer 58 of polyimide or the like.
Close to the intersections of the scan lines 52 and signal lines 54 there are formed thin film transistors (TFTs) that serve as switching elements. For each TFT, a semiconductor layer 59 is laid between a scan line 52 and signal line 54; a signal line portion on the semiconductor layer 59 constitutes the TFT's source electrode S and a scan line portion below the semiconductor layer 59 constitutes the gate electrode G, while the drain electrode D is constituted by a part of the pixel electrode 56 that overlaps part of the semiconductor layer 59.
The color filter substrate CF has a configuration such that a color filter layer 61, overcoat layer 62 and alignment layer 63 are provided on a surface of a second transparent substrate 60. To form the IPS mode liquid crystal display panel 50A, the array substrate AR and color filter substrate CF are positioned opposing each other so that the pixel electrode 56 and common electrode 55 on the array substrate AR and the color filter layer 61 on the color filter substrate CF face each other, liquid crystal LC is sealed therebetween, and polarizing plates 64 and 65 are deposed on the outer side of substrates AR and CF respectively, placed so that their polarization directions cross over each other.
In this IPS mode liquid crystal display panel 50A, when an electric field is formed between the pixel electrode 56 and common electrode 55, the liquid crystals, which are aligned horizontally, will gyrate horizontally as shown in FIGS. 11 and 12. By means of this it is possible to control the amount of incident light from the backlight that is transmitted. This IPS mode liquid crystal display panel 50A has the advantages of a wide viewing angle and high contrast, but also has the problems of low aperture ratio and transmittance because the common electrodes 55 are formed from the same metallic material as the common wires 53 or scan lines 52, as well as the problem of color variation depending on the viewing angle.
FFS mode liquid crystal display panels (see JP-A-2002-14363 and JP-A-2002-244158) have been developed in order to resolve the problems of low aperture ratio and transmittance in IPS mode liquid crystal display panels. The operating principles of such an FFS mode liquid crystal display panel will now be described using FIGS. 13 to 15. FIG. 13 is a schematic plan view of a single pixel portion of an FFS mode liquid crystal display panel, FIG. 14 is a cross-sectional view along line XIV-XIV in FIG. 13, and FIG. 15 is a cross-sectional view along line XV-XV in FIG. 13.
This FFS mode liquid crystal display panel 70A has an array substrate AR and a color filter substrate CF. The array substrate AR has multiple scan lines 72 and common wires 73 provided parallel with one another on a surface of a first transparent substrate 71, and multiple signal lines 74 provided in the direction crossing over these scan lines 72 and common wires 73. A common electrode (also termed “opposed electrodes”) 75 connected to the common wires 73 and formed from indium tin oxide (ITO) or a like transparent material is provided so as to cover each space delimited by the scan lines 72 and signal lines 74, and over a surface of the common electrode 75 there are provided, with a gate insulator 76 interposed, a pixel electrode 78A constituted of ITO or a like transparent material, in which there are formed multiple stripe-like slits 77A. The surfaces of the pixel electrode 78A and the multiple slits 77A therein are covered by an alignment layer 80.
Close to the positions where the scan lines 72 and signal lines 74 intersect there are formed TFTs that serve as switching elements. For each TFT, a semiconductor layer 79 is laid on a surface of a scan line 72, and a portion is extended from a signal line 74 so as to cover part of the semiconductor layer 79's surface and constitute the TFT's source electrode S; a scan line portion below the semiconductor layer 79 constitutes the gate electrode G, while a part of the pixel electrode 78A that overlaps part of the semiconductor layer 79 constitutes the drain electrode D.
The color filter substrate CF has a configuration such that a color filter layer 83, overcoat layer 84, and alignment layer 85 are provided on a surface of a second transparent substrate 82. To form the FFS mode liquid crystal display panel 70A, the array substrate AR and color filter substrate CF are positioned opposing each other so that the pixel electrode 78A and common electrode 75 on the array substrate AR and the color filter layer 83 on the color filter substrate CF face each other, liquid crystal LC is sealed therebetween, and polarizing plates 86 and 87 are deposed on the outer side of substrates AR and CF respectively, placed so that their polarization directions are orthogonal to each other.
In this FFS mode liquid crystal display panel 70A, when an electric field is formed between the pixel electrode 78A and common electrode 75, the field is oriented toward the common electrode 75 at both sides of the pixel electrode 78A, as shown in FIGS. 14 and 15, and consequently, not only does the liquid crystal present at the slits 77A move, but so does the liquid crystal present over the pixel electrode 78A. As a result, The FFS mode liquid crystal display panel 70A has the features of having an even wider viewing angle and higher contrast than the IPS mode liquid crystal display panel 50A, and moreover an ability to provide bright displays thanks to possessing high transmittance. In addition, the FFS mode liquid crystal display panel 70A has a greater overlap area, viewed from above, between the pixel electrode 78A and common electrode 75 than has the IPS mode liquid crystal display panel 50A, and, as a collateral effect thereof, a larger holding capacity and hence the advantage that no auxiliary capacity line needs to be specially provided.
In an FFS mode liquid crystal display panel, similarly to the case of an IPS mode liquid crystal display panel, it is preferable for the sake of the display characteristics that the rubbing direction should be orthogonal to the signal lines, and the pixel electrodes be provided at a slight inclined angle relative to the rubbing direction. Accordingly, a structure may be adopted whereby stripe-like slits 77B provided in a pixel electrode 78B are inclined relative to the scan lines 72 or common wires 73 as in the FFS mode liquid crystal display panel 70B shown in FIG. 16. Similarly, in order to eliminate color variation depending on the viewing angle, the stripe-like slits 77C provided in a pixel electrode 78C may be arranged in two mutually inclined sets, one above the other, thus producing dual domains, as in the FFS mode liquid crystal display panel 70C shown in FIG. 17. Further, the signal lines 72 may be provided in a crank-shape in a direction orthogonal to the scan lines 74, and the multiple common electrodes and pixel electrodes 78D be arranged in a delta layout, so that the black matrices provided on the color filter substrate at the portions opposed to the signal lines 72 will not form straight lines, and the device will be capable of image displays in which the black matrices are inconspicuous, as in the FFS mode liquid crystal display panel 70D shown in FIG. 18.
The FFS mode liquid crystal display panels 70B and 70C shown in FIGS. 16 and 17 differ from the FFS mode liquid crystal display panel 70A shown in FIG. 13 only in that the slits 77B and 77C provided in their pixel electrodes 78B and 78C are inclined. Moreover, the FFS mode liquid crystal display panel 70D shown in FIG. 18 differs from the FFS mode liquid crystal display panel 70A shown in FIG. 13 only in that slits 77D provided in its pixel electrodes 78D are inclined and that its multiple common electrodes and pixel electrodes 78D are arranged in a delta layout. Below therefore, component elements that have identical structure to those in the FFS mode liquid crystal display panel 70A shown in FIG. 13 are assigned the identical reference numerals and detailed descriptions thereof are omitted.
Also in the IPS mode liquid crystal display panel 50A shown in FIG. 10, it is possible, similarly with the FFS mode liquid crystal display panels 70B and 70C described above, and as in the IPS mode liquid crystal display panel 50B shown in FIG. 19, to improve the display quality by making the alignment layer's rubbing direction cross the signal lines 54 and providing a pixel electrode 55B and a common electrode 56B at a slight inclined angle relative to the rubbing direction; and further possible, as in the IPS mode liquid crystal display panel 50C shown in FIG. 20, to eliminate color variation depending on the viewing angle, by arranging a pixel electrode 55C and a common electrode 56C each to be inclined in a different extension direction, leftward or rightward, thus producing dual domains. In addition, although an illustration thereof is omitted among the drawings, it is also possible with such IPS mode liquid crystal display panels 50A, 50B, 50C to arrange the multiple pixel electrodes 55, 55A, 55B and common electrodes 56, 56A, 56B in a delta layout so as to obtain an image display in which the black matrices are inconspicuous, as in the FFS mode liquid crystal display panel 70D shown in FIG. 18. Moreover, the IPS mode liquid crystal display panels 50B and 50C shown in FIGS. 19 and 20 differ from the IPS mode liquid crystal display panel 50A shown in FIG. 10 only in that their pixel electrodes 55B, 55C and common electrodes 56B, 56C are inclined. Below therefore, component elements that have identical structure to those in the IPS mode liquid crystal display panel 50A shown in FIG. 10 are assigned the identical reference numerals and detailed descriptions thereof are omitted.
Thus, FFS mode liquid crystal display panels have the features of having an even wider viewing angle and higher contrast than IPS mode liquid crystal display panels, and moreover of being able to provide bright displays thanks to possessing high transmittance. Furthermore they can be driven with low voltage, and what is more, have a larger holding capacity generated as collateral effect, which means that they yield good display quality without special provision of auxiliary capacity lines.
However, it is well known that when used for prolonged periods, liquid crystal display panels are prone to the phenomenon of burn-in. This is the case both with IPS mode liquid crystal display panels and with FFS mode liquid crystal display panels. But it has been found that the burn-in phenomenon occurs more markedly in related art FFS mode liquid crystal display panels such as described above than in related art IPS mode liquid crystal display panels. The present inventors inferred, as a result of many and varied investigations into the causes of the burn-in phenomenon occurring more markedly in the FFS mode liquid crystal display panels than in the IPS mode liquid crystal display panels, that—it being the case that the electrical field produced by the large signal voltages applied to the scan lines affects the alignment of the nearby liquid crystals—a contributory factor is that whereas in the IPS mode liquid crystal display panels the path of the electrical force lines oriented from the pixel electrode toward the liquid crystals and the path of the electrical force lines oriented from the liquid crystals toward the scan lines are symmetrical, in the FFS mode liquid crystal display panels they are asymmetrical.
More precisely, in IPS and FFS mode liquid crystal display panels, the voltage applied to the scan lines is approximately −10V in the state when given pixels are deselected, and approximately +15V in the state when selected, but since the duration for which given pixels are selected is extremely short, DC voltage of approximately −10V is applied over extended periods. In the case of an IPS mode liquid crystal display panel however, as is plain from FIG. 12, an electrical force line E1 oriented from the pixel electrode 56 to the scan line 52 enters the liquid crystal layer LC via the pixel electrode 56, protective insulator 57, and alignment layer 58, and from the liquid crystal layer LC reaches the scan line 52 via the alignment layer 58 and protective insulator 57; thus, the force line's path in traveling from the pixel electrode 56 to the liquid crystal layer LC is symmetrical with the path thereof in traveling from the liquid crystal layer LC to the scan line 52.
In an FFS mode liquid crystal display panel by contrast, as is plain from FIG. 15, an electrical force line E2 oriented from the pixel electrode 78A to the scan line 72 enters the liquid crystal layer LC via the pixel electrode 78A and alignment layer 80, but from the liquid crystal layer LC reaches the scan line 72 via the alignment layer 80 and gate insulator 76. Thus, the force line's path in traveling from the pixel electrode 78A to the liquid crystal layer LC is asymmetrical with the path thereof in traveling from the liquid crystal layer LC to the scan line 72. As a result, in the FFS mode liquid crystal display panel the pixel electrode and/or the alignment layer on the surface thereof are more prone than in the IPS mode liquid crystal display panel to be irreversibly affected by the DC field arising from the signals applied to the scan lines 72. This is inferred to be the reason why the burn-in phenomenon occurs more markedly in the FFS mode liquid crystal display panel than in the IPS mode liquid crystal display panel.